MONTGOMERY, Ala. — The once-unimaginable Republican defeat in Alabama’s special Senate election on Tuesday tore open divisions between the party’s establishment and populist wings, inciting bitter recriminations in the Republican Party as Democratic enthusiasm surges — especially in the nation’s cities and affluent suburbs.

While the accusations of child molestation and teenage sexual abuse made Roy S. Moore a uniquely poor candidate, it was not lost on Republicans that Democrats, black and white, had flooded to the polls here just over a month after voters in Virginia overwhelmingly rejected Republican candidates.

In Alabama, a state that Donald J. Trump won by 28 points last year, exit polls showed that as many voters disapproved of the president as approved of him, an ominous sign for Republicans that revealed both soaring Democratic intensity and growing dissatisfaction with Mr. Trump among moderate voters.

“The side that has the energy and anger and enthusiasm usually prevails,” said Representative Charlie Dent, Republican of Pennsylvania. “And Democrats don’t have to be for anything, they just have to be against us.”

Republicans are now bracing for the possibility of another unexpectedly difficult special election, in March, this one in a conservative-leaning House district in western Pennsylvania, and they are resigned to having to spend money to protect what has been a safe seat. Further, Mr. Dent, who has already said he will not seek re-election next year, confirmed he has had conversations with TV news executives about becoming an analyst, raising the possibility that he would leave his seat early and create yet another special election for his party. (“I have no definitive plans,” he said.)

And the seemingly unremitting stream of accusations from women about male lawmakers committing sexual misconduct is raising the possibility of even more unexpected resignations, which could at a minimum cause Republicans to have to direct more cash toward races for which they had not planned.

For Republicans, competing in the first midterm elections of a president this unpopular was always going to be difficult. But that natural disadvantage is being exacerbated by the conflict between Senate Republicans and antiestablishment conservatives such as Stephen K. Bannon, Mr. Trump’s former chief strategist who backed Mr. Moore and encouraged the president to join the fight.

Republican lawmakers were all but gleeful on Wednesday at Mr. Bannon’s comeuppance, ridiculing him for supporting the rare Republican who could lose an Alabama Senate race.

“I don’t think Steve Bannon had a good night,” said Senator Richard C. Shelby of Alabama, the veteran lawmaker who helped torpedo Mr. Moore’s candidacy by publicly refusing to vote for him.

Mr. Moore’s backers “aren’t the face of the Republican Party that I know,” he added.

Yet with their majority now reduced to a single seat, Senator Mitch McConnell of Kentucky, the majority leader, and his lieutenants remain uneasy about what antiestablishment forces could do in next year’s primary races and have identified at least two additional campaigns where they believe Bannon-backed candidates could weaken the party.

Mr. McConnell’s allies plan to intervene as aggressively as needed in Arizona, where Kelli Ward, a far-right former state legislator, is seeking the seat that will be vacated by Senator Jeff Flake, and in Nevada, where Danny Tarkanian, a perennial candidate who has branded himself as a Trump cheerleader, is challenging Senator Dean Heller in a Republican primary race.

Both candidates, McConnell advisers said, would be completely intolerable as general election nominees.

Complicating matters further for Republicans in Arizona is the health of Senator John McCain, who has an aggressive form of brain cancer and is struggling with the side effects of his treatment. Mr. McCain has made clear he will leave the Senate and return to his home state if he is not able to carry out his duties. But if Arizona’s governor fills the seat by appointing Representative Martha McSally, an all but declared candidate, it could deprive Republicans of perhaps their most formidable candidate against Ms. Ward and further imperil their prospects for retaining the seat held by Mr. Flake, who is retiring.

Establishment-aligned Republicans are still anxious about whether Chris McDaniel, a hard-line Mississippi state senator, plans to challenge Senator Roger Wicker in a primary race next year. Should Mr. McDaniel, who narrowly lost to Senator Thad Cochran in 2014, claim the nomination, it could energize black Mississippians in the same fashion as black Alabamians on Tuesday.

Mr. McDaniel said Wednesday that he was “leaning toward the Senate race,” but he would not rule out the possibility of instead running for lieutenant governor in 2019.

He criticized Mr. Wicker for being “against our state flag,” an emblem that features the Confederate flag, and said he would make his decision in mid-January. (He previously said he would decide in October.)

The Republicans’ Senate woes predate the rise of both Mr. Bannon and Mr. Trump. The party has lost five Senate races since 2010 because of nominees who were far outside the political mainstream, including two before Mr. Moore who were defeated after making damaging comments about rape.

It remains to be seen how wide a battlefield Mr. Bannon and his affiliates will even contest in 2018. At the Senate level, beyond Mississippi and Nevada, there are scant opportunities for Mr. Bannon to pursue incumbent Republican senators. And internal polling for Mr. Wicker has found him comfortably ahead in a primary race.

Further, Mr. Bannon has privately started to dial back some of his ambitions, suggesting to allies that Mr. Tarkanian, his ostensible pick to challenge Mr. Heller in Nevada, also might be better off running for a House seat that will be open next year.

Eric Beach, who leads a pro-Trump group, Great America PAC, which backed Mr. Moore, said it was increasingly likely that in a number of states, the populist wing of the Republican Party and the party establishment would settle on the same candidate, including in Missouri and Ohio, where Democratic senators are seeking re-election.

But Mr. Beach, who is also advising Ms. Ward in Arizona, stressed that the party should seek out recruits who can “appeal to the masses,” and said that Republican lawmakers should embrace Ms. Ward rather than pressure her to stand down.

Mr. Bannon is unlikely to target Senator Orrin G. Hatch of Utah, who is deciding whether to retire or to seek an eighth term, because Mr. Hatch has become close with Mr. Trump, and the president has urged him to run again.

And if Mr. Hatch does retire, his state could easily become a triumph for the traditional Republican establishment: Mitt Romney, a former presidential candidate and one of Mr. Trump’s most insistent Republican critics, continues to tell allies that he would most likely seek the seat if Mr. Hatch steps down. Mr. Romney would be exceedingly difficult to beat in a Utah primary race — though Mr. Hatch, according to a longtime adviser, has told Mr. Romney that he is not “sure he was ready to move on just yet.”

Given the possibility of damaging tumult on the Republican side, Democrats have tried to recruit credible Senate candidates even in long-shot states. Had Doug Jones not entered the race in Alabama, well before it appeared competitive, the party could have been left without a credible opponent for Mr. Moore once he emerged as the Republican nominee.

In Utah, for instance, a Democratic member of the Salt Lake County Council, Jenny Wilson, filed to challenge Mr. Hatch. And in Wyoming, where Mr. Bannon has conferred with Erik Prince, a wealthy military contractor, about challenging Senator John Barrasso in a Republican primary race, Gary Trauner, a businessman and former congressional candidate, has also entered the campaign as a Democrat.

More worrisome for the Republican Party are states that are likely to prove competitive in a general election, such as Tennessee, where former Gov. Phil Bredesen, a Democrat, buoyed his party’s leaders by entering the contest to replace Senator Bob Corker, who is retiring.

Mr. Bredesen said in an interview that he was confident he could win — “I have no desire to go on a suicide mission at this point in my life” — and argued that “the wind is at my back.”

“People continue to get more and more frustrated with the approach the current administration is taking,” he said.

And if other potential Republican Senate recruits are daunted by the forbidding political environment, it could hamper their ability to win some of the Democratic-controlled seats they have been eyeing for months. In Florida, for example, advisers to Gov. Rick Scott said he was mindful of the midterm climate and was not yet sold on challenging Senator Bill Nelson.

It is the House, though, where Republicans could bear the brunt of a backlash to Mr. Trump next year. Democrats need 24 seats to capture the majority, and officials in both parties increasingly believe that may be attainable.

“History tells us the House is in jeopardy,” said Corry Bliss, who runs a “super PAC” dedicated to House races. “Last night is a reminder that candidates matter, there’s no votes in pedophilia and next year we need to nominate strong candidates who can raise money.”